| 摘要 |
A power control device of an electric tool with an open or closed loop control apparatus which adjusts the power fed to the electric motor of the tool by means of pulse width modulation. A frequency generator is controlled to vary the width of the pulses of power applied to the motor as a function of the required power output. The figure shows that as more power is required by the motor, the width of the pulses p1 to p12 increases in terms of the period, a1 to a12, and the frequency of the pulses is reduced. A small period and high frequency is selected at low motor speeds to reduce the perceptibility of the noise arising from pulse width modulation, and as the power requirements increase the frequency is decreased to reduce power losses in the transistors.
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